Tara had seen Jack get left behind in her dreams. She had felt the way the TARDIS had felt, had known that he was a fact and that that was wrong. That he shouldn't have existed. By giving her power to Rose, she had let such a thing come to be. But she didn't want to accept it. She'd been as much at fault as the Doctor had -- she had accepted the flight path out of Satellite Five.
She hadn't seen the end of the universe yet, though she suspected why they'd ended up there. Tara would have tried to shake him off. It was against everything she'd known as the TARDIS to let him board her.
"I'm sorry, Jack," was all she could say. Because she knew he felt wronged, and that he was a bit emotional about it all. He'd need to think on it a bit longer. He'd probably had hundreds of years in his dream life to consider it. Perhaps he wouldn't have that time here, but humans were amazing things sometimes.